Flash capable cameras, both film-based and digital electronic, typically employ a flash tube enclosing an ionizable gas, such as xenon, to create the bright flash of light needed to capture images in a low light level scene. Basic elements of a flash circuit used to fire the tube typically include a battery, a voltage converter flash charging circuit, a flash voltage storage capacitor and a trigger circuit connected to a trigger terminal associated with the flash tube. The flash charging circuit converts the low battery voltage of about 1.5 v or 3.0 v to a high flash charge voltage on the order of 330 v. This voltage is applied to the flash storage capacitor which is coupled to the input electrodes of the flash tube. The gas in the tube, in its normal state, presents an extremely high impedance which prevents the flash capacitor from discharging through the tube to create any bright flash light emission. When it is desired to take a picture, the trigger circuit responds to opening of the shutter to generate and apply to the trigger terminal an intermediate high voltage, on the order of 4,500 v that is sufficient to ionize the gas molecules in the tube. This reduces the impedance of the tube to a very low level allowing the flash capacitor to discharge the stored flash voltage through the low impedance of the flash tube thereby creating the desired intense flash of light.
A commonly used form of trigger circuit employs a trigger capacitor connected through a high impedance to the flash charging circuit and the flash storage capacitor to be charged to the same voltage as the flash storage capacitor. The trigger capacitor is also coupled in a circuit leading through the primary winding of a trigger voltage step-up transformer and a normally open trigger switch, the switch being actuated by the camera shutter mechanism to be closed when the shutter is opened. The secondary winding of the transformer is coupled to the trigger terminal of the flash tube. When the user initiates a picture-taking sequence by depressing the shutter release button, the trigger switch is closed causing the trigger capacitor to discharge through the primary of the trigger transformer which, in turn, generates the high voltage pulse on the trigger terminal needed to fire the flash tube as described above.
Other forms of flash trigger circuits have been described that use a piezoelectric device to generate the high voltage pulse for ionizing the gas in the flash tube. In an early patent U.S. Pat. No. 4,025,817, issued May 24, 1977 entitled “Trigger Device for an Electronic Flash Unit” and assigned to the assignee of the present invention, a mechanically actuated piezoelectric device is substituted in the trigger circuit for the trigger capacitor and step-up trigger transformer. The arrangement described includes a hammer and anvil that mechanically deforms the piezoelectric crystal to generate the output pulse applied to the flash tube trigger terminal. While effective, the arrangement requires added relatively complex and costly mechanical structure to actuate the piezoelectric crystal. Additionally, added circuit components are required to assure reliability of triggering of the flash tube.
In issued patent U.S. Pat. No. 6,564,015 B2, granted May 13, 2003, a flash circuit is described that utilizes an electronically driven piezoelectric transformer device both to charge the flash storage capacitor and to trigger the flash tube. Unlike the mechanically actuated device, a piezoelectric transformer operates in response to a voltage pulse voltage or an oscillatory voltage applied to input terminals to generate a stepped up output voltage pulse or oscillatory voltage. In prior art FIG. 7 of this patent, a pair piezoelectric transformers are each driven by an oscillatory circuit and a driver circuit to generate separately the desired output voltages. In the remaining circuits disclosed, a single piezoelectric transformer device is driven by an oscillatory circuit and a driving circuit. The voltage outputs are alternately switched to charge the flash charge capacitor and trigger firing of the flash tube, thereby accomplishing both functions with a single piezoelectric device. However, as shown by the several embodiments in the patent, limitations of the transformer device force the use of added circuit components and/or modification of the transformer output electrode connections with the transformer crystal to achieve the necessary dual functionality thereby adding cost and complexity.